1. Field of the Invention
This invention relates to a frequency synthesizer, and more particularly to a frequency synthesizer for which a high speed frequency hopping operation is required.
2. Description of the Related art
Various frequency synthesizers are conventionally known, and an exemplary one of such conventional frequency synthesizers is shown in FIG. 3. Referring to FIG. 3, the conventional frequency synthesizer shown includes a phase frequency detector 7. The phase frequency detector 7 inputs a reference signal 6 and the output of a variable frequency divider 8, effects phase frequency detection and outputs pulse signals having pulse widths corresponding to differences in phase and frequency between the input signals to a phase frequency to voltage converting circuit 2. The phase frequency to voltage converting circuit 2 outputs a voltage corresponding to the pulse widths of the pulse signals inputted thereto to a low-pass filter 11. The low-pass filter 11 removes high frequency components of the signal inputted thereto and passes the other components of the input signal therethrough. A voltage-controlled oscillator 10 inputs the signal having passed through the low-pass filter 11 and outputs a signal having a frequency corresponding to the voltage of the signal inputted thereto to the variable frequency divider 8. The output of the voltage-controlled oscillator 10 also makes a synthesizer output 5 of the frequency synthesizer. A dividing ratio generating circuit 9 outputs a signal for controlling the dividing ratio of the variable frequency divider 8 in accordance with frequency data 4 inputted thereto to the variable frequency divider 8.
With the conventional frequency synthesizer, a high speed frequency hopping operation seems to be realized by such means as to raise the loop gain of a feedback system or to widen the passing frequency band of the low-pass filter 11. However, when any of such means is provided actually, if the hopping speed is raised, then this will result in deterioration in stability in phase and frequency in a phase frequency locked condition and also in spurious output. Accordingly, the conventional frequency synthesizer is disadvantageous in that it is difficult to achieve a high speed frequency hopping operation.